Process and apparatus for making mineral fibres

ABSTRACT

Particulate mineral material suitable for forming a fiberisable melt is melted in a flame formed by combustion of powdered carbonaceous fuel with preheated air and the particulate mineral material is then preheated, and the exhaust gases are subjected to NOx reduction, in a cyclone preheater ( 22 ).

This application is a United States national filing under 35 USC 371 ofinternational (PCT) application No. PCT/EP02/07062, filed Jun. 26, 2002,claiming priority to: U.S. Provisional Application No. 60/301,754, filedJun. 27, 2001; GB Application, 0115760.1, filed 27 Jun. 2001; and EPOapplication 01310090.4, filed 3 Dec. 2001.

This invention relates to the production of mineral fibres by burningcombustible material in the presence of inorganic particulate materialand thereby forming a melt, and then fiberising this melt to form thefibres.

When the fibres are glass fibres, the melt is typically formed by addinginorganic particulate material to a preformed pool of melt in anelectric or other tank furnace. This is appropriate having regard to thechemistry, physical properties and economics of the manufacture of glassfibres, typically having a chemical analysis, by weight of oxides, ofabove 10% Na₂O+K₂O, below 3% iron as FeO, below 20% CaO+Mgo, above 50%SiO₂ and below 5% Al₂O₃, and often also some boron. However this systemis not practical nor economic, having regard to the melt temperature,other physical properties and economics, for the manufacture of rock,stone or slag fibres, typically having an analysis, by weight of oxides,of below 10% Na₂O+K₂O, above 20% CaO+MgO above 3% iron as FeO, and below50% SiO₂ and, often, above 10% Al₂O₃, and usually boron in, at most,trivially small amounts.

The normal way of producing the melt for slag, stone or rock fibres isby means of a shaft furnace in which a self-supporting stack ofinorganic particulate material is heated by combustion of combustiblematerial in the furnace. The stack gradually melts and is replenishedfrom on top, with melt draining down the stack and out from the bottomof the furnace. The normal furnace for this purpose is a cupola furnace.

It is necessary for the stack to be self-supporting and permeable to thecombustion gases, which are generally generated by combustion ofcarbonaceous material in the stack. It is therefore necessary thateverything in the stack is relatively coarse (in order that the stack ispermeable) and has high physical strength and does not collapse untilcombustion or melting is well advanced. In practice this means that thecarbonaceous material is coke and the particulate material is eithercoarsely crushed rock, stone or slag or is in the form of briquettesformed from fine particulate material.

Accordingly, if the material which is available is only available infinely divided form, it is necessary to incur the expense andinconvenience of forming it into briquettes. Briquetting usually usessulphur-containing materials as binder, such as Portland cement withgypsum, and this means that the effluent is liable to have a highsulphur content, which has to be treated. The gas would typicallycontain H₂S and CO if it is not subjected to after-burning.

For this, and other reasons, it is generally necessary to subject theeffluent gases from the cupola furnace to an after-burning stage, inorder that the gases which are discharged into the atmosphere areenvironmentally satisfactory, and it would be desirable to be able toavoid the need for using an after-burner.

The cupola or other stack furnace system also has the disadvantage thatconditions in the furnace always tend to be sufficiently reducing thatsome of the iron is reduced to metallic iron. This necessitatesseparating metallic iron from the melt, reduces wool production, leadsto the provision of iron waste and also tends to incur the risk ofcorrosion in the zone containing iron and slag.

Another disadvantage is that the process does not have high thermalefficiency.

Despite these disadvantages, the process using a cupola or other stackfurnace has been widely adopted throughout the world for the manufactureof rock, stone or slag fibres, e.g., having the analysis given above.

Nevertheless, it would clearly be desirable, and has been desirable fora long time, to devise a system which avoids some or all of thesedisadvantages. Thus the invention aims to provide a system which hashigh thermal efficiency and which provides an environmentallysatisfactory effluent, preferably without the use of an after-burner orother special pollution-controlling effluent treatment. It is alsodesirable that the system does not result in reduction of iron and doesnot necessitate briquetting.

Almost twenty years ago U.S. Pat. No. 4,365,984 proposed the manufactureof slag, stone or rock fibres by an entirely different process. Thisinvolves suspending powdered coal in preheated combustion air andcombusting the suspended coal in the presence of suspended particulatemineral material in a circulating combustion chamber, i.e., a combustionchamber in which the suspended particulate materials and air circulatein a system which is or approaches a cyclone circulation system.

This process results in the formation of a mineral melt and hot exhaustgases. The melt is collected in a tank and a stream of melt is thenfiberised by centrifugal fiberising apparatus. The hot exhaust gases areutilised for preheating the combustion air, before it is mixed with thecoal, by heat exchange between air and the exhaust gases. In thisprocess, the combustion air which is mixed with the coal and theparticulate material is described as having a temperature between 430and 650° C. and the flame temperature in the furnace is described asbeing between 1500 and 1900° C. Preferably some or all of the inorganicparticulate material is provided as part of the suspended coal, as aresult of using waste coal tailings from a fine coal washing circuit.

Although the process would, in theory, be operable, and would avoid theneed of briquetting and would probably eliminate the risk of reductionof iron, the process as described is clearly subject to majorenvironmental effluent problems and is of low efficiency. Accordingly,in practice it is neither economically nor environmentally competitivewith the conventional shaft furnace processes and so the circulatingcombustion chamber technology has not been developed for the manufactureof slag or rock fibres. This is despite the fact that there have beennumerous publications of circulating combustion chamber technology forvarious mineral products in the intervening twenty years.

One particular environmental effluent problem which is likely to ariseis the presence of NOx in the exhaust gases. The reducing conditions ina cupola tend to minimise this problem but the less reducing conditions,and in particular the described oxidising conditions, that would prevailin the circulating combustion chamber are liable, at the hightemperatures of the process, to result in the effluent gases containinga significant amount of NOx, and this would create a major environmentalproblem.

It would be desirable to be able to avoid this and other environmentalproblems of processes using non-reducing conditions in the combustionchamber, and to avoid the various technical and economic andenvironmental problems associated with cupola and other shaft furnaces.

According to the invention, we provide a process for making a mineralmelt which can be used for making mineral fibres and which comprises

-   -   suspending powdered carbonaceous fuel in preheated combustion        air and combusting the suspended carbonaceous fuel to form a        flame,    -   suspending in the flame mineral material which has been        preheated to at least 700° C. and melting the mineral material        in a circulating combustion chamber and thereby forming mineral        melt and hot exhaust gas,    -   separating the hot exhaust gas from the melt and collecting the        melt,    -   contacting the exhaust gas from the melt in a cyclone preheater        under NOx reducing conditions with the particulate mineral        material which is to be preheated and melted and thereby        reducing NOx in the exhaust gas and providing the particulate        mineral material which has been preheated to at least 700° C.,    -   and preheating the combustion air by heat exchange of air with        the exhaust gas from the cyclone preheater.

The invention includes the described process of making the melt whereinthe collected melt is then taken as a stream to a fiberising apparatus,usually a centrifugal fiberising apparatus, and is fiberised to fibreswhich are then collected, for instance as a web and converted intobonded or other mineral wool products in conventional manner. Thecomposition of the melt is generally such that the fibres are of thetype which are conventionally described as slag, stone or rock fibres.

The invention also includes processes in which the collected melt can beused for some entirely different purpose, for instance for themanufacture of cast products.

The invention also includes the plant which is used for making the melt,such as the means for forming the flame and for feeding the particulatemineral material into the flame and the circulating combustion chamberfor this, and the recycling system including the cyclone preheater.

It is easily possible to operate the process so that it is economicallyand environmentally advantageous compared to conventional processesusing a shaft furnace. In particular, it is possible to operate theprocess in a cost-effective manner to provide melt which is free ofreduced iron and exhaust gases which are substantially free of NOx andother undesirable impurities or which have a level of contaminationwhich is sufficiently low that it is environmentally acceptable.

The NOx reducing conditions are preferably generated by including in thecyclone preheater nitrogenous material which will reduce NOx under theconditions prevailing in the preheater. The nitrogenous material may beincluded in the hot exhaust gas which is fed to the preheater or may beadded direct to the preheater.

The nitrogenous material which is included in the preheater cyclone ispreferably ammonia or ammonium compound, an amine or urea, wherein theurea may be free or, more preferably, is a resinous product such as aurea formaldehyde or phenol urea formaldehyde resin. It is particularlypreferred that the NOx reducing conditions are generated by including inthe particulate material waste bonded mineral wool which is fed to thepreheater cyclone, wherein the waste bonded mineral wool contains a urearesin (usually phenol urea resin) and/or ammonia or an ammonium compound(for instance as a buffering agent for resin in the waste wool). Thus,by the invention, it is possible simultaneously to utilise wastematerial and to react it under appropriate conditions so as to reduce asignificant amount of the NOx in the exhaust gases to nitrogen.

The amount of ammonia or ammonia derivative or other NOx-reducingcompound is preferably 1 to 4 (preferably 1-2 or, especially, 1-1.7)moles per mole NOx and the reaction is preferably conducted at atemperature of 800° C. to 1050° C. The reaction residence time ispreferably at least 0.3. seconds and most preferably at least 1 second.Typically this can be the residence time of the particulate mineralmaterial in the cyclone preheater, and/or the ducting, until the exhaustgas is cooled below reaction temperature, e.g., below 800° C. Underthese conditions, preferably with a temperature in the range 800 to1050° C., substantially all the NOx is reduced to nitrogen, even thoughthe atmosphere in the preheater is preferably oxidising.

Thus, according to another preferred feature of the invention thegaseous atmosphere in the cyclone preheater contains excess oxygen,preferably in an amount of at least 1% or 2% and most preferably atleast 4%, for instance up to 8% by volume. by weight of the gaseousatmosphere. Despite the oxidising nature of the atmosphere, NOx isreduced by the added ammonia or other nitrogenous compound under theconditions defined for the preheater.

The preheater can thus simultaneously operate as a NOx reducer and anoxidising after-burner to burn pollutants such as hydrogen sulphide andcarbon monoxide from the circulating combustion chamber.

Preferably the exhaust gases which are separated from the melt and whichare then fed to the cyclone preheater contain less oxygen than theamount which is present in the cyclone preheater and so preferably airor other source of oxygen is added to the exhaust gases either in thepreheater or between the melt and the preheater.

Preferably the combustion in the circulating combustion chamber is nearstoichiometric or even sub-stoichiometric. As a result of this, theamount of NOx generated during the combustion is minimised. The ratio ofoxygen to combustible material is generally from 0.8 to 1, mostpreferably 0.85 to 0.99, often around 0.92 to 0.97.

Thus, in preferred process and apparatus according to the invention thecombustion of the carbonaceous particulate material and the melting ofthe particulate mineral material is conducted under slightlysub-stoichiometric conditions and the exhaust gas from this is thenadjusted to be slightly oxidising and the exhaust gases are then, in asingle operation, both subject to oxidising after burning and to NOxreduction, in a cyclone preheater.

The temperature of the exhaust gases when they are separated from themelt is preferably 1400 to 1700° C., often 1500 to 1600° C. Thetemperature of the gases entering the cyclone preheater is generally inthe range 1000 to 1500° C. When, as is normal, this temperature is lessthan the temperature of the gas when it leaves the melt, the reductionin temperature can be achieved by dilution with air and/or liquidammonia. The proportions of the inflowing exhaust gas and theparticulate mineral material should be such that the mineral material ispreheated to the desired temperature, typically 700° or 800 to 1050° C.,in the cyclone preheater.

The exhaust gases from the preheater cyclone are used for preheating theair for the combustion of the carbonaceous material and generally thegases have a temperature in the range 800 to 900° C. when they emergefrom the preheater cyclone. They are preferably used for heat exchangewith the incoming combustion air so as to preheat that air to atemperature of at least 500° C. and preferably 600 to 900, mostpreferably around 700 to 800° C.

The carbonaceous material which is used as the fuel can be anyparticulate carbonaceous material that has a suitable calorific value.This value can be relatively low, for instance as low as 10000 kJ/kg oreven as low as 5000 kJ/kg. Thus it may be, for instance, dried sewagesludge or paper waste. Preferably it has higher calorific value and maybe spent pot liner from the aluminium industry, coal containing wastesuch as coal tailings, or powdered coal.

The fuel and air is preferably such that the adiabatic flame temperature(i.e., the temperature that would be achieved from the fuel and air ifthere is no exchange of enthalpy with particulate mineral material orother surroundings) is in the range 1800° C. to 2500° C. or more,preferably in the range 2000 to 2500° C.

It is desirable to start the combustion of the carbonaceous material inthe preheated air before adding the preheated particulate material intothe flame so as to allow the flame temperature to become relatively highbefore the cooler particulate mineral material is added, since otherwiseefficiency may be reduced significantly. Preferably the flametemperature is at least about 1000° C. and preferably at least 1200° C.before the preheated mineral material is added to it. However if theflame temperature is too high there will be increased production of NOxand so preferably the flame temperature is not above 1500° C. or 1600°C. at the time when the particulate mineral material is added.

In general, materials and conditions are preferably such that themaximum temperature in the circulating combustion chamber and in thegases emerging from it is not more than 1600° C.

The invention is described below by reference so the accompanyingdrawings in which

FIG. 1 is a flow-diagram showing one form of apparatus and methodaccording to the invention and

FIG. 2 is a diagram illustrating an alternative arrangement to replacethe combustion chamber 25 and tank 8 in FIG. 1.

Powdered coal from a screw feed 1 or other feeder is injected intopreheated combustion air from duct 2 using an injector 3.

The powdered coal in screw feed 1 may be coal fines but preferably some,and usually at least 50% and preferably at least 80% and usually all ofthe coal is made by milling lump coal, for instance using a ball mill 4,wherein the lump coal may be supplied from a silo 5. The coal, whetherit is supplied initially as fines or lump, may be good quality coal ormay be waste coal containing a high inorganic content, for instance 5 to50% inorganic with the balance being carbon. Preferably the coal ismainly or wholly good quality coal for instance bituminous orsub-bituminous coal (ASTM D388 1984) and contains volatiles whichpromote ignition.

The coal or other carbonaceous fines which are injected into thepreheated combustion air preferably have a particle size in the rangefrom 50 to 1000 μm, preferably about 50-200, generally about 70 μmaverage size, with the range being 90% below 100 μm.

The preheated combustion air preferably has a temperature of from 500 to800° C., most preferably 700° C. at a time when it is contacted with thepowdered coal.

The resultant stream of coal suspended in air passes along duct 24typically at a velocity of 20-40 m/s, and enters a circulatingcombustion chamber 25. One or more gas burners 6 may be provided atconvenient places, for instance as shown in the flow diagram and/or induct 24, to initiate combustion if this is necessary.

Particulate inorganic material is supplied by feeder 7 into the streamof powdered coal suspended in air in the duct 24.

The pressure in the combustion chamber 25 is usually higher than thepressure in the cyclone 22 and so it is necessary for the feeder 7 to beconstructed to insure that the solids flow downwardly despite theincrease in pressure. For instance the feeder 7 may comprise a screwfeed which discharges through a weighted pressure valve or it maycomprise a fluidised bed valve.

It is necessary to ensure that combustion of the coal has initiatedbefore the particulate inorganic material is added, as otherwiseinferior and inefficient combustion may occur. In practice thetemperature of the flame is generally at least 1200° C. and preferablynot more than 1500° C. at the point where the particulate material isfed into the flame.

The particulate inorganic material is usually fed into the flame quiteclose to the circulating combustion chamber 25. In practice thereforethe feeder 7 is usually close to the inlet to the chamber 25 and it mayeven be direct into the chamber 25.

It is not essential to have the horizontal duct 24 for the establishmentof the flame since it is possible, by appropriate design of the chamberinlets, to inject the coal and preheated air direct into the chamber. Itis also possible to feed the particulate material direct into thechamber at a position such that, the flame temperature is sufficientlyhigh before the mineral particulate material contacts the flame.

The circulating combustion chamber 25 is of the type which is frequentlyreferred to as a cyclone furnace. Preferably it is water cooled. Theconstruction of suitable cyclone furnaces is described in variouspatents. including U.S. Pat. Nos. 3,855,951, 4,135,904, 4,553,997,4,544,394, 4,957,527, 5,114,122 and 5,494,863.

Within the circulating combustion chamber 25, combustion of theparticulate coal continues and the particulate mineral material isconverted to melt while suspended in air. Melt and particulate materialmay be thrown on to the walls of the chamber and will flow down thechamber, predominantly as melt.

The circulating chamber 25 can be a horizontal or inclined-cyclone butoften is vertical. It may lead downwardly into a tank for collectingmelt. Preferably the chamber opens direct into the tank without goingthrough a conical or other restricted exit duct of the type which isconventional in many systems, since providing a conical duct as the exithas no advantage and impedes flow from the base of the chamber.

The tank can be in the base of the chamber (for instance as described inU.S. Pat. No. 4,553,997) or it can be a settling tank 8 of considerablyenhanced volume, as shown in FIG. 1. The settling tank 8 should havesufficient gas volume to allow for precipitation of the melt dropletsfrom the exhaust gas and sufficient melt volume to ensure dissolution ofparticles, which may only be partly melted, and for homogenisation ofthe melt There may be a gas burner 6 or other means for applying extraenergy to the settling tank if necessary, for instance to raise thetemperature of the exhaust gases, especially during start up.

Melt is run off from the tank, when appropriate, through gutter 9 as astream and may then be subjected to fiberisation in conventional manner,for instance using a cascade spinner or a spinning cup or any otherconventional centrifugal fiberising process. Alternatively it may be runoff to some other manufacturing process, e.g., a casting process.

Exhaust gases free of melt are taken from the circulating combustionchamber 25 or from the settling tank 8 into which the chamberdischarges. They are taken direct from this chamber through duct 10.

Most or all of the particulate material which is to be melted ispreheated by the exhaust gases, typically by being fed in particulateform into the flowing stream of exhaust gas in duct 10 by the feed 11,and the resultant suspension in gas is then passed into the preheatercyclone 22.

The particle size of the mineral material which is fed into the exhaustgases by the feed 11 is preferably in the range 0 to 10 mm, usually 0 to4 mm, preferably 0 to 2 mm.

The rate of flow of the exhaust gas when the particulate material issuspended in them is generally in the range 10 to 40 m/s. He velocitiesrefer to the dimension of the main tube, but the velocities may furtherbe increased just at the feeding point by inserting a venturi section,whereby the velocity may reach values of 100 m/s or even more. Theparticulate material may be fed into the exhaust gas as it approachesthe cyclone preheater, or in the cyclone preheater.

The particulate material which is fed in through feed 11 is suppliedfrom hoppers 12 and 13, wherein hopper 13 is particularly importantbecause it contains waste particulate material which contains a sourceof nitrogen, such as bonded mineral wool wherein the bonding agent is aurea resin. The various materials from hoppers 12 and 13, withadditional pulverisation in a ball mill or other mill if necessary, arethen blended and fed into a silo 14 and from this they are continuouslydischarged into the feed 11.

The exhaust gases in duct 10 approaching feed 11 will usually have beencooled down by dilution with air and/or ammonia (not shown) to atemperature of 1200° C. to 1500° C. suitable for preheating theparticulate material in the cyclone 22 to a temperature in the range 700to 1000° C., generally around 800° C.

These exhaust gases usually leave the cyclone 22 at a temperature in therange 800 to 1000° C., preferably around 900° C. At these temperature,there will be selective non-catalytic reduction of NOx predominantly tonitrogen, with the result that the exhaust gases from the cyclone 22,which leave through duct 15, will have a satisfactorily low NOx contentand will preferably be substantially free of NOx.

They then pass through, a heat exchanger 16 by which there is indirectheat exchange with combustion air from ventilator 17, thus generatingthe desired stream of preheated combustion air through duct 2. The wastegas is vented through ventilator 27, and filter 18 to stack 19.

In the modification illustrated diagramatically in FIG. 2, the chamber25 and tank 8 is replaced by a water cooled conical cyclone combustionchamber 28 having a relatively small collecting zone 29 at its baseleading to a controllable gutter 9 for the discharge of melt. There is atangential inlet 30 into the cyclone for the introduction of powderedcoal or other particulate fuel and preheated air direct from injector 3(in which event the flame is established within the cyclone 28) or froma duct 24 (in which event the flame will be established, at leastpartly, before entering the inlet 30). The feeder 7 discharges thepreheated inorganic particulate material through one or more inlets 26and 27 positioned in the cyclone chamber 28 such that the flame isestablished to an adequate temperature before it meets the inorganicmaterial. Exhaust gas is taken off from the cyclone through outlet 10.

1. A process for making mineral fibres comprising providing a preheatedcombustion air, suspending a powdered carbonaceous fuel in saidpreheated combustion air and combusting the suspended carbonaceous fuelto form a flame, providing particulate mineral material which has beenpreheated to at least 700° C., suspending said preheated particulatemineral material in said flame and melting the mineral material in acirculating combustion chamber at an adiabatic flame temperature of atleast 1800° C. and thereby forming a mineral melt and hot exhaust gases,separating said hot exhaust gases from said mineral melt and collectingthe melt, flowing a stream of said collected melt to a centrifugalfiberising apparatus and forming mineral fibres by centrifugallyfiberising the stream of melt, wherein the preheating of the particulatemineral material is carried out by conveying said exhaust gases to acyclone preheater and contacting said exhaust gases in said cyclonepreheater under NOx-reducing conditions with (a) said particulatemineral material which is to be melted and (b) waste bonded mineral woolcontaining a source of nitrogen, and thereby both reducing NOx in theexhaust gases and preheating said particulate material to at least 700°C., removing the exhaust gases with reduced NOx from said cyclonepreheater, and wherein said combustion air is preheated by heat exchangeof air with said exhaust gases with reduced NOx removed from the cyclonepreheater, and wherein the hot exhaust gases separated from the mineralmelt have their temperature reduced from the temperature at the timethey are separated from the mineral melt by dilution with at least oneof air and liquid ammonia while being conveyed to the cyclone preheaterand before contact with the particulate mineral material in the cyclonepreheater.
 2. A process according to claim 1 in which said cyclonepreheater contains oxygen.
 3. A process according to claim 1 in whichthe combustion is conducted under substoichiometric oxygen to fuelconditions.
 4. A process according to claim 1 in which said NOx reducingconditions comprise a temperature of 700° C. to 1050° C. and thepresence of said source of nitrogen.
 5. A process according to claim 4in which said cyclone preheater temperature is in the range of 800° C.to 1050° C.
 6. A process according to claim 1 in which said circulatingcombustion chamber is a conical cyclone combustion chamber having anaxial outlet for the exhaust gas from its top and an inlet for at leastone of said powdered fuel, said preheated air, and said flame, saidinlet being disposed non-radially into the top of the cyclone and anoutlet for mineral melt from its base, and exhaust gas is removed viathe axial outlet, at least one member selected from the group consistingof said powdered fuel, said preheated air and said flame are introducedvia said inlet, and said melt is removed from said outlet for themineral melt.
 7. A process according to claim 6 in which said combustionchamber temperature is in the range of 800° C. to 1050° C.
 8. A processaccording to claim 1 in which said preheated particulate mineralmaterial is fed into the combustion chamber and is suspended in theflame in the combustion chamber.
 9. A process according to claim 1 inwhich said source of nitrogen is selected from the group consisting ofurea resin, ammonia and an ammonia compound.
 10. A process according toclaim 9 in which said source of nitrogen comprises a urea resin bondingagent for the waste bonded mineral wool.
 11. A process according toclaim 1 in which said exhaust gases temperature is reduced to at least1500° C.
 12. A process according to claim 11 in which said exhaust gasestemperature reduction is effected by dilution of said exhaust gases withair.